1. Field of the Invention
This invention relates to a CCD (Charge Coupled Device) solid state image sensor.
2. Discussion of the Related Art
In general, electron-hole pairs generated in response to incident light, CCD-solid state image sensors have selected, accumulated, and transferred only one side of the pairs (i.e., either electrons or holes) as signal charges, while quickly removing the other unselected signal charges as unnecessary charges. Since the mobility of an electron is greater than that of a hole, the electron is generally the signal charge.
FIG. 1 illustrates a block diagram of a frame transfer type CCD-solid state image sensor. The frame transfer type CCD-solid state image sensor includes a sensing area 11 for generating signal charges in response to incident light, a storage area 12 for storing the signal charges from the sensing area 11 for a certain period, an HCCD (Horizontal Charge Coupled Device) 13 for extracting the stored signal charges from the storage area 12 line by line, a charge detection and amplification circuit 14 for detecting and amplifying signal charges transferred from the HCCD 13.
The sensing area 11 of the frame transfer type CCD-solid state image sensor generates signal charges in response to incident light and accumulates them for a certain time period.
The signal charges accumulated in the sensing area 11 are transferred to the storage area 12 at one time during a vertical erasing period for a storage therein. The signal charges are extracted and transferred to the HCCD 13 line by line before the next vertical erasing period. Upon the extraction of the stored signal charges from the storage area 12, the aforementioned operation is repeated, i.e., the accumulated signal charges in the sensing area are transferred to the storage area again.
Because the signal charges are electrons, a signal detection and amplification circuit adapted for electrons is used as the signal detection and amplification circuit 14.
FIG. 2 is a sectional illustration of the storage area 12 in the frame transfer type CCD-solid state image sensor. Though a sectional illustration of the storage area 12 in the frame transfer type CCD-solid state image sensor has only been shown, the sensing area 11 and the storage area 12 have almost identical sections, except the storage area 12 has a light-shielding layer of aluminum to shield incident light and a transfer electrode, while sensing area 11 does not so that it may generate signal charges in response to incident light.
Referring to FIG. 2, the storage area 12 in the frame transfer type CCD-solid state image sensor includes an n.sup.- type substrate 121, a p.sup.- type well 122 formed in the n.sup.- type substrate 121, an n type buried channel region 123 formed in the p.sup.- type well 122, channel stop regions 124 formed in the p.sup.- type well 122 in contact with the n type buried channel region 123, an insulation film 125 formed of SiO.sub.2 on the substrate 121, and a transfer electrode 126 formed on the insulation film 125.
FIGS. 3A and 3B are sectional illustrations across lines 2A-2A' and 2B-2B', respectively, and FIG. 3C shows potential profiles across lines 2A-2A' and 2B-2B'.
As shown in FIG. 3C with line 132, the signal charges generated in the sensing area 11 in response to incident light are accumulated in the n type buried channel region 123. As shown in FIG. 3C with a line 131, the holes are gathered in the p.sub.+ type channel stop regions 124 through an interface between the substrate 121 and the insulation film 125, i.e., Si/SiO.sub.2, and dissipated through structures around them.
As has been explained, the holes are taken as unnecessary signal charges. The holes have not been considered in the CCD-solid state image sensor except to remove them within a short period so that they do not affect the transfer of the electrons.
FIG. 4 illustrates a graph showing photoelectric transfer characteristics of the CCD-solid state image sensor, with the CCD surface illuminance on the X-axis and the CCD output voltage on the Y-axis. A dynamic range of a CCD is generally defined as tile linear range where the illuminance and the output voltage are proportional, and a sensitivity of a solid state image sensor is represented by the slope (mV/LUX) of the linear range.
Therefore, as shown in FIG. 4, a solid state image sensor with a sensitivity characteristic of A has twice the sensitivity of a solid state image sensor with a sensitivity characteristic of B. However, in view of the illuminance dynamic range, the solid state image sensor B has a wider dynamic range than the solid state image sensor A.
In order to match the dynamic range of sensor B, the maximum voltage of sensor A should be doubled. However, doubling the voltage is not practical because of limitations on both the driving voltage of the CCD and the input voltage to a video signal processing circuit.
Even if there was a technique that could detect charges with a sensitivity 10 times greater, the aforementioned problem of illuminance dynamic range in utilizing the technique would still exist. That is, even though the highly sensitive charge detection technique would be useful under very low illuminance, in the case of a picture with sharp contrast, or of a very bright outdoor landscape under daylight, the contrast becomes indistinct for portions with illuminances greater than a certain amount which become completely white.
Matching both the driving voltage and the signal processing circuit voltage to a maximum output voltage predetermined to be 10 times greater is theoretically and actually not possible.
As a result, a solid state image sensor that can detect and amplify charges with high sensitivity in low illuminance regions and with low sensitivity in high illuminance regions is needed.